Chemokines are chemotactic cytokines that are released by a wide variety of cells, to attract cells such as leukocytes (including macrophages, T-cells, eosinophils, basophils, neutrophils and myeloid-derived suppressor cells) and endothelial cells to sites of inflammation and tumor growth. There are two main classes of chemokines, the CXC-chemokines and the CC-chemokines. The class depends on whether the first two cysteines are adjacent (CC-chemokines), or are separated by a single amino acid (CXC-chemokines). There are currently at least 17 known CXC-chemokines, which include but are not limited to CXCL1 (GROα), CXCL2 (GROβ), CXCL3 (GROγ), CXCL4 (PF4), CXCL5 (ENA-78), CXCL6 (GCP-2, CXCL7 (NAP-2), CXCL8 (IL-8, NAP-1), CXCL9 (MIG) and CXCL10 (IP-10). There are currently at least 28 known CC chemokines, which include but are not limited to CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β), CCL5 (RANTES), CCL7 (MCP-3), CCL8 (MCP-2), CCL-11 (eotaxin-1) and CCL20 (MIP-3α). Individual members of the chemokine families are known to be bound by at least one chemokine receptor, with CXC-chemokines generally bound by members of the CXCR class of receptors, and CC-chemokines by members of the CCR class of receptors. For example, CXCL8/IL-8 is bound by the receptors CXCR1 and CXCR2.
Since CXC-chemokines often promote the accumulation and activation of neutrophils, these chemokines are implicated in a wide range of acute and chronic inflammatory disorders such as psoriasis, rheumatoid arthritis, radiation-induced fibrotic lung disease, autoimmune bullous dermatoses (AIBD), chronic obstructive pulmonary disease (COPD) and ozone-induced airway inflammation (see, Baggiolini et al., FEBS Lett. 307:97 (1992); Miller et al., Crit. Rev. Immunol. 12:17 (1992); Oppenheim et al., Annu. Rev. Immunol. 9: 617 (1991); Seitz et al., J. Clin. Invest. 87: 463 (1991); Miller et al., Ann. Rev. Respir. Dis. 146:427 (1992); and Donnely et al., Lancet 341: 643 (1993), Fox & Haston, Radiation Oncology, 85:215 (2013), Hirose et al., J. Genet. Syndr. Genet. Ther. S3:005 (2013), Miller et al., Eur. J. Drug Metab. Pharmacokinet. 39:173 (2014), Lazaar et al., Br. J. Clin. Pharmacol., 72:282 (2011)).
A subset of CXC chemokines, those which contain the ELR motif (ELR-CXC), have been implicated in the induction of tumor angiogenesis (new blood vessel growth). These include the CXCR2 ligand chemokines CXCL-1, CXCL2, CXCL3, CXCL5 and (Strieter et al. JBC 270: 27348-27357 (1995)) Some CXCR2 ligand ELR-CXC chemokines are exacerbating agents during ischemic stroke (Connell et al., Neurosci. Lett., 15:30111 (2015). All of these chemokines are believed to exert their actions by binding to CXCR2. Thus, their angiogenic activity is due to their binding and activation of CXCR2 expressed on the surface of vascular endothelial cells (ECs) in surrounding vessels.
Many different types of tumors are known to produce ELR-CXC chemokines, and production of these chemokines correlates with a more aggressive phenotype (Inoue et al. Clin Cancer Res 6:2104-2119 (2000)) and poor prognosis (Yoneda et. al. J Nat Cancer Inst 90:447-454 (1998)). As ELR-CXC chemokines are potent chemotactic factors for EC chemotaxis, they probably induce chemotaxis of endothelial cells toward their site of production in the tumor. This may be a critical step in the induction of tumor angiogenesis. Inhibitors of CXCR2 will inhibit the angiogenic activity of the ELR-CXC chemokines and therefore block the tumor growth. This anti-tumor activity has been demonstrated for antibodies to CXCL8 (Arenberg et al. J Clin Invest 97:2792-2802 (1996)), ENA-78 (Arenberg et al. J Clin Invest 102:465-72 (1998)), and CXCL1 (Haghnegahdar et al. J. Leukoc Biology 67:53-62 (2000)).
Many tumor cells express CXCR2 and tumor cells may thereby stimulate their own growth by secreting ELR-CXC chemokines. Thus, in addition to with decreasing angiogenesis within tumors, CXCR2 inhibitors may directly inhibit the growth of tumor cells.
CXCR2 is often expressed by myeloid-derived suppressor cells (MDSC) within the microenvironment of tumors. MDSC are implicated in the suppression of tumor immune responses, and migration of MDSC in response to CXCR2 ligand chemokines is most likely responsible for attracting these cells into tumors. (see Marvel and Gabrilovich, J Clin. Invest. 13:1 (2015) and Mackall et al., Sci. Trans. Med. 6:237 (2014). Thus, CXCR2 inhibitors may reverse suppressive processes and thereby allow immune cells to more effectively reject the tumor. In fact, blocking the activation of CXC-chemokine receptors has proven useful as a combination therapy with checkpoint inhibitors in suppressing tumor growth, suggesting that CXCR2 blockade may also enhance tumor rejection in combination with other anti-tumor therapies, including but not limited to vaccines or traditional cytotoxic chemotherapies (see Highfill et al., Science Translational Medicine, 6:237 (2014)).
Hence, the CXC-chemokine receptors represent promising targets for the development of novel anti-inflammatory and anti-tumor agents.
There remains a need for compounds that are capable of modulating activity at CXC-chemokine receptors. For example, conditions associated with an increase in IL-8 production (which is responsible for chemotaxis of neutrophil and T-cell subsets into the inflammatory site and growth of tumors) would benefit by compounds that are inhibitors of IL-8 receptor binding.